Solar batteries in currently practical use comprise a flat pn junction formed by diffusing impurities in a flat semiconductor wafer. The solar batteries having this structure produce maximum output when light enters the light receiving surface at a right angle. Output decreases as light enters the light receiving surface at smaller angles. These solar batteries have a strong directional pattern. It would be difficult to say that they constantly utilize light in an efficient manner. Furthermore, wafers are produced by slicing a semiconductor crystal ingot and, thus, there are significant processing losses including margins to cut, which leads to increased production costs.
U.S. Pat. No. 4,581,103 discloses a solar battery element that is produced by melting and dropping a highly pure metal silicon material to form p-type crystal particles and diffusing n-type impurities in the p-type crystal to form a spherical pn junction and a solar battery module produced by connecting those solar battery elements using aluminum foil. The spherical solar battery elements of the solar battery module do not have individual electrodes before being assembled into a module, and are mechanically pressed into pores formed in a sheet of aluminum foil, electrically connecting the n-type surface. Then, the part of the n-type layer surface of the solar battery element that protrudes downward from the pore is removed by, for example, etching, to expose the p-type silicon or the core, causing the p-type silicon to make contact with another sheet of aluminum foil to form a positive electrode. A number of solar battery elements having a pn junction are connected in this way to form a module in which multiple solar battery elements are given electrodes and connected in parallel by two sheets of aluminum foil. In producing a solar battery module in this way electrodes are formed and connected in parallel concurrently using two sheets of aluminum foil. However, the p-type region is exposed after the n-type layer is connected to the aluminum foil, making it is difficult to evaluate the properties and quality of individual solar battery elements. Furthermore, this structure only allows for parallel connection. Another solar battery module must be connected in order to increase the output voltage. When the solar battery elements have a smaller diameter, the distance between the two aluminum foil sheets is decreased, making it difficult to insulate the aluminum foil from each other, and complicating the production process. The positive and negative electrodes are formed below the center of the solar battery element; in other words, they are formed at asymmetrical positions. This causes several disadvantages. For example, sufficiently improved photoelectric conversion efficiency is not available because the electric current between the positive and negative electrodes is localized at points where the distance between the electrodes is smaller. The aluminum foil blocks light and, therefore, only the light receiving surface above the aluminum foil is useful. Light from all directions is not received and, therefore, the output is not increased.
Japanese Laid-Open Patent Publication H09-162434 discloses a solar battery sheet in which multiple spherical solar battery elements are supported by a glass fiber cloth formed by weaving vertically extended conductive wires and horizontally extended glass fibers. In such a solar battery, the solar battery elements are supported by conductive wires, by which they are easily insulated from each other.
However, also in the solar battery elements used in the solar battery described in Japanese Laid-Open Patent Publication H09-162434, the n-type layer is connected to a negative electrode conductive wire, exposing the p-type region which is entirely surrounded by the n-type layer and connected to a positive electrode wire. Only the n-type layer is externally exposed before the conductive wires and solar battery elements are connected, making it so that the individual solar battery elements cannot be inspected before being connected, with the same problems as exist in the afore-mentioned citations. The positive conductive wire connected to the p-type region is also connected to the n-type layer. Then, the n-type layer is irradiated with light for electrochemical etching to separate the pn junction, by which the positive electrode wire is connected only to the p-type region. Solar battery elements are etched at different rates, making it difficult to reliably separate the pn junction in all the solar battery elements.
The solar battery element of this publication also has the same problem as the afore-mentioned citations because it is connected to the positive and negative electrode conductors asymmetrically about the center, with the disadvantage that, when the solar battery elements are replaced with spherical light emitting diodes, spherical light emitting diodes cannot be used because they emit light in a limited region between the conductive wires and fail to emit light in all directions.
In WO98/15983, the inventor of the present application proposed multiple spherical elements that are solar battery elements or light emitting devices and a light receiving or emitting module sheet in which the spherical elements are connected. The spherical element comprises a spherical p-type (or n-type) single crystal semiconductor (such as silicon), an n-type (or p-type) diffused layer formed near the surface of the single crystal semiconductor, a nearly spherical pn junction, and a pair of negative and positive electrodes provided opposite to each other about the center of the spherical single crystal semiconductor. A number of these spherical elements are arranged in a matrix of multiple rows and multiple columns and are connected in series and/or in parallel to constitute a light receiving or emitting module sheet.
The spherical elements position the electrodes at opposite positions to one another about their center. It is easy to connect multiple spherical elements in series by arranging the positive and negative electrodes of adjacent spherical elements to make direct contact with each other. However, it is not easy to connect spherical elements in parallel.
The inventor of the present application provided a resolution to this problem in WO03/017382 in which two parallel conductive wires are used to flank and connect in parallel the positive and negative electrodes of spherical elements arranged with their electrodes aligned to form a column of spherical elements and, the conductive wires of the adjacent columns of spherical elements then being connected to connect the columns of spherical elements in series.
The light receiving or emitting module sheet has the problem that its tensile strength is high in the lengthwise direction of the conductive wires, but is significantly lower in a direction orthogonal thereto. Further, it is necessary to simplify the connection between the spherical elements and conductive wires and improve productivity.
Objects of the present invention include providing a light receiving or emitting module sheet that may be constituted only by good spherical elements, a light receiving or emitting module sheet that has a high tensile strength, a light receiving or emitting module sheet that yields a high photoelectric or electrophoto conversion rate using spherical elements, and a light receiving or emitting module sheet that is easy to produce. Other objects of the present invention will apparent from the description of the embodiments of the present invention.